1. Field of the Invention
The present invention relates to a novel porous article to which a sulfur containing atomic group is introduced, i.e., a sulfur containing atomic group introduced porous article, a process for introducing a sulfur containing atomic group onto outer-inner surfaces of a porous article, and a battery separator comprising the sulfur containing atomic group introduced porous article.
2. Description of the Related Art
Hitherto, a sulfonating treatment was known as one process for introducing a sulfur containing atomic group onto a surface of a porous article. The conventional sulfonating treatments include, for example, a treatment with a fuming sulfuric acid, or a treatment with a plasma produced by an atmospheric-pressure glow discharge in an atmosphere of a gaseous mixture of a treating gas and a rare gas. The sulfonation on the surface of the porous article, i.e., the introduction of sulfonic groups (SO.sub.3 H) onto the surface to be treated, can impart a hydrophilic property to the porous article or enhance a hydrophilic property of the porous article. A degree of the sulfonation on the treated porous article correlates with a ratio (S/C) of the number of sulfur atoms (S) to the number of carbon atoms (C) on the treated surface (hereinafter referred to as a surface S/C ratio). A higher degree of sulfonation correlates with a higher surface S/C ratio.
The sulfonation treatment with a fuming sulfuric acid can introduce sulfonic groups at a high degree, and a treated article having a high surface S/C ratio can be obtained. Nevertheless, the sulfonated article treated in this process tends to exhibit a lower ratio (O/C) of the number of oxygen atoms (O) to the number of carbon atoms (C) on the treated surface (hereinafter referred to as a surface O/C ratio), i.e., an index to show a degree of introduction of a carboxyl group and so on. The surface O/C ratio correlates with a wettable property, or wetting property. An article with a lower surface O/C ratio has a poor wettable property. For example, when a sulfonated material obtained in this process is used as a battery separator, a further treatment (for example, an application of a surface active agent) is required to enhance the wettable property. Further, the process using a fuming sulfuric acid requires a large-scale apparatus for washing away unreacted fuming sulfuric acid, and a large-scale treatment of a waste liquor containing a concentrated sulfuric acid. There is also a problem of a deterioration in the strength of the treated article.
The treatment with a plasma produced by an atmospheric-pressure glow discharge in an atmosphere of a gaseous mixture of a treating gas and a rare gas is disclosed in, for example, Japanese Unexamined Patent Publication (Kokai) No. 9-85026. The publication discloses a process for treating a porous article having a three-dimensional network wherein the article is placed between a pair of electrodes which are located opposite to each other, at least one of the electrodes carrying a dielectric layer on a surface facing the other electrode; and a discharged plasma treatment is conducted under an atmospheric pressure in the presence of a gaseous mixture of a treating gas and a rare gas. The publication description also states that if a concentration of the treating gas in the gaseous mixture is beyond 10% by volume of the rare gas, i.e., a concentration of the treating gas is beyond 9.09% by volume of the gaseous mixture, it becomes difficult to generate a uniformly discharged plasma upon applying a voltage, and thus the concentration of the treating gas is preferably 0.01 to 9.09% by volume of the gaseous mixture. Further, the publication mentions that a discharged plasma treatment was actually performed under a sulfur dioxide/helium mixture containing 0.5% by volume of sulfur dioxide as a treating gas, and states that sulfur dioxide, sulfur trioxide, oxygen or nitrogen can be used as the treating gas.
As apparent from the description of the above publication, it is necessary in the process for an atmospheric-pressure glow discharge to apply a voltage in the presence of more than 90% by volume of rare gas, to generate a uniformly discharged plasma. Therefore, it is impossible to increase the concentration of the treating gas in the gaseous mixture over the limitation, and thus, a sulfonated article having a high surface S/C ratio and a high surface O/C ratio at the same time was not known.